Traveling wave amplifiers (TWA) are commonly employed for amplification of signals in ultra-wideband applications. The bandwidth of a TWA may start from almost DC (e.g., a few kilohertz) and extend up to microwave frequencies and even millimeter-wave frequencies.
FIG. 1 shows a simplified schematic diagram of one typical four-section TWA 100. TWA 100 includes an input port 110, a plurality of first inductance elements 120, a plurality of second inductance elements 130, a plurality of transistors 140, a first load 150, a second load 160, and an output port 170. As shown in FIG. 1, pairs of first inductance elements 120 may be combined into third inductance elements 180, and pairs of second inductance elements 130 may be combined into fourth inductance elements 190. In that case, third inductance elements 180 each have an inductance that is twice the inductance of each first inductance element 120, and fourth inductance elements 190 each have an inductance that is twice the inductance of each second inductance element 130. Other elements such as blocking capacitors, power supply voltages, bias voltage connections, etc. are omitted from the simplified schematic diagram of FIG. 1 for ease of illustration.
In TWA 100, first and third inductance elements 120 and 180 form a synthetic gate line 125 between input port 110 and first load 150. Similarly, second and fourth inductance elements 130 and 190 form a synthetic drain line 175 between second load 160 and output port 170. Meanwhile, transistors 140 are connected between synthetic gate line 125 and synthetic drain line 175, with a gate of each transistor 140 connected between adjacent first inductance elements 120 of synthetic gate line 125, a drain of each transistor 140 connected between adjacent second inductance elements 130 of synthetic drain line 175, and a source of each transistor 140 connected to ground.
To better understand the operating principles of TWA 100, FIG. 2 shows an equivalent circuit of each transistor 140. With the source grounded, in an ideal (lossless case), it can be seen that there is a first capacitance Cgs between the gate and ground, and a second capacitance, Cds between the drain and ground. In that case, TWA 100 can be modeled as including an artificial gate transmission line 300 and an artificial drain transmission line 350 as shown in FIG. 3. Artificial gate transmission line 300 has a characteristic impedance:
(1) ZO=(LG/CG)1/2, where LG is the inductance of third inductance element 180 and CG is the combination of the gate capacitance Cgs and the parasitic capacitance of third inductance element 180. Similarly, artificial drain transmission line 350 has a characteristic impedance:
(2) ZO=(LD/CD)1/2, where LD is the inductance of fourth inductance element 190 and CD is the combination of the drain capacitance Cds and the parasitic capacitance of fourth inductance element 190.
Given n stages, the gain of TWA 100 is:
(3) Gain=(n2*gm*ZO2)/4, where gm is the transconductance gain of transistor 140.
Furthermore, the cutoff frequency, Fc(gate), of artificial gate transmission line 300 is:Fc(gate)=2/(LG*CG)1/2   (4)and the cutoff frequency, Fc(drain), of artificial drain transmission line 350 is:Fc(drain)=2/(LD*CD)1/2.   (5)
In general, Cgs>>Cds, and therefore CG>>CD. So, from equations (4) and (5), we can see that the upper cutoff frequency of TWA 100 is set by CG. That is, as CG increases, the upper cutoff frequency (bandwidth) of TWA 100 decreases, and vice versa.
So, to increase the bandwidth of TWA 100, it becomes necessary to reduce CG. This can be accomplished by using transistors 140 having smaller gate widths, reducing Cgs and thereby CG. However, in general transistors having a smaller gate width also have a reduced gain and reduced power-handling capacity. Accordingly, in general, to increase the bandwidth of TWA 100, gain and power output capabilities of TWA 100 must be sacrificed. This is undesirable, and unacceptable in some applications.
To increase the bandwidth of a TWA, several alternative TWA configurations have been proposed.
FIG. 4 shows a simplified schematic diagram of a portion of an alternative TWA 400. TWA 400 includes a parallel resistor/capacitor (RC) circuit 410 disposed between the gate of each transistor 140 and the synthetic gate line 125. The construction and operation of TWA 400 is the same as TWA 100 other than the parallel RC circuit 410, so for clarity of explanation, a description of the remaining elements and the operation will not be repeated, and only the differences between TWA 400 and TWA 100 will be described.
The addition of parallel RC circuit 410 in series with the gate-source capacitance Cgs of transistor 140, reduces the capacitance CG of the artificial gate transmission line of TWA 400 compared to the TWA 100 of FIG. 1. Therefore, the cutoff frequency Fc(gate) of the artificial gate transmission line TWA 400 is increased with respect to TWA 100. Accordingly, all other things being equal, the bandwidth of TWA 400 is increased compared to TWA 100.
FIG. 5 shows a simplified schematic diagram of a portion of an alternative TWA 500. TWA 500 includes cascode transistors 540 and 545 connected between synthetic gate line 125 and synthetic drain line 175 in place of transistor 140 of TWA 100. The construction and operation of TWA 500 is the same as TWA 100 other than the substitution of cascode transistors 540 and 545 in TWA 500 for the single transistor 140 in TWA 100, so for clarity of explanation, a description of the remaining elements and the operation will not be repeated, and only the differences between TWA 500 and TWA 100 will be described.
The cascode transistors 540 and 545 present a reduced capacitance to synthetic gate line 125 compared to transistor 140 in FIG. 1. As a result, the capacitance CG of the artificial gate transmission line of TWA 500 is reduced compared to TWA 100 of FIG. 1. Therefore, the cutoff frequency Fc(gate) of the artificial gate transmission line of TWA 500 is increased with respect to TWA 100. Accordingly, all other things being equal, the bandwidth of TWA 500 is increased compared to TWA 100. Furthermore, the effective output impedance of the cascode transistors 540 and 545 as seen by synthetic drain line 175 is increased compared to TWA 100.
FIG. 6 shows a simplified schematic diagram of a portion of yet another alternative TWA 600. TWA 600 represents a sort of combination of the features of TWA 400 and TWA 500. More specifically, TWA 600 includes the cascode transistors 540 and 545 of TWA 500, and the RC circuit 410 of TWA 400. By adding RC circuit 410 to the cascode transistor arrangement of FIG. 5, the bandwidth of TWA 600 can be increased compared to the bandwidth of TWA 500.
However, the above-described TWA configurations of FIGS. 4-6 have certain shortcomings. For example, all other things being equal, in general TWA 400 suffers from a reduced gain across the band compared to TWA 100. Furthermore, gain compensation at the lower end of the frequency band is difficult in TWA 400. Meanwhile, TWA 500 can extend the bandwidth somewhat compared to TWA 100, but it is still not enough for many applications. Also, TWA 500 is quite prone to oscillation. The bandwidth of TWA 500 can be extended further by adding the RC circuit 410 of TWA 400, as shown by TWA 600, but this presents the same disadvantages of TWA 400—namely, the gain is reduced across the band, and gain compensation at the lower end of the frequency band becomes more difficult.
What is needed, therefore, is a traveling wave amplifier which can provide increased bandwidth, with a higher gain, greater power handling capability, and adequate stability.